The present invention relates a color-separating and -recombining optical system having polarization beam splitters and a projection display using the optical system.
Color projection displays operate as follows: White light is separated into three primary colors R (Red), G (Green) and B (blue). The separated color components are guided to the corresponding spatial light modulators (abbreviated to SLM hereinafter) for optical modulation in accordance with a video signal. The modulated color components are recombined and projected onto a screen, thus displaying a color image thereon.
Color projection displays are classified into three types in accordance with SLMs to be used, such as, a type with SLMs, another with reflecting SLMs, and still another with a DMD (Digital Mirror Device).
Compact projection displays with transparent SLMs and DMDs having relatively simple optical architecture are available but have difficulty in resolution.
On the contrary, reflective SLMs exhibit high resolution but pose a problem in compactness due to complex optical system using this type of SLMs. Particularly, projection displays equipped with reflective SLMs require polarization beam splitters (abbreviated to PBS hereinafter) for splitting light beams incident to the SLMs and reflected light beams that have been modulated by the SLMs. In detail, each reflective SLM requires two or more of PBSs for high contrast, thus resulting in complex optical architecture for reflective projection displays.
Colorlink Inc. (US) has proposed a color-separating and -recombining optical system having no problem on optical architecture in use of reflective LSMs, introduced in literature xe2x80x9cHigh Contrast Color Splitting Architecture Using Color Polarization Filtersxe2x80x9d by Michael G. Robinson et., SID 00 DIGEST, 92-95(2000).
FIG. 1 is a plan view illustrating an optical architecture for a projection display 300 using reflective SLMs, proposed by Colorlink Inc.
A color-separating and -recombining optical system 290 (enclosed by a dot line) has cubic- or square column-like first to fourth PBSs 102, 103, 104 and 105 arranged such that polarization-splitting planes 121, 131, 141 and 151 intersect each other almost like the character xe2x80x9cXxe2x80x9d.
First wavelength-selective polarizing converters (G-phase plates) 106 and 107 are provided on the light-incident plane side of the first PBS 102 (the left side of the PBS 102 in FIG. 1) and light-emitting plane side of the fourth PBS 105 (the right side of the PBS 105 in FIG. 1), respectively, for rotating the plane of polarization of a G-linearly-polarized light by 90 degrees.
Second wavelength-selective polarizing converters (R-phase plates) 108 and 109 are provided between the first and the third PBSs 102 and 104, and the third and the fourth PBSs 104 and 105, respectively, for rotating the plane of polarization of a R-linearly-polarized light by 90 degrees.
Linearly-polarized light is classified into S-polarized light and P-polarized light. A polarized light is decided as S- or P-polarized light in accordance with relativity between its plane of polarization and a polarization-splitting plane of a PBS to which it is incident. In other words, a polarized light is called S-polarized light when its plane of polarization is orthogonal to an incident plane against a polarization-splitting plane of a PBS, whereas it is called P-polarized light when its plane of polarization is horizontal to the incident plane.
The projection display 300 has a relatively simple optical architecture for high contrast even though it requires three PBSs for each reflective SLM.
Nonetheless, this projection display has a problem of low contrast at the corners of a black image screen due to birefringence caused by a transparent material for the PBSs due to wrong choice for the transparent material in the projection display 300 when a high-intensity discharge lamp of 100W or more is used.
Japanese-Unexamined Patent Publication No. 9-54213 discloses that a transparent material of 1.5xc3x9710xe2x88x928 cm2/N or less as the absolute value of opto-elastic constant is suitable for such PBSs.
It is disclosed that a transparent material of low opto-elastic constant is suitable at least for a main (reflective) PBS that splits incident light and light reflected therefrom after modulation.
The inventors of the present invention have, however, found that the problem discussed above cannot be solved by employing such transparent material of low opto-elastic constant when it is used only for the main PBSs (the second and the third PBSs 103 and 104) for the projection display 300 equipped with the color-separating and -recombining optical system 290.
The above problem could be solved by employing such transparent material of low opto-elastic constant when it is used for all of the four PBSs, which, however, results in high cost for the color-separating and -recombining optical system.
Such transparent material of low opto-elastic constant is generally several times or several ten times more expensive than usual optical glass such as BK7 because it contains much lead and hence too weak and soft for machining.
Moreover, the color-separating and -recombining optical system 290, offered by Colorlink Inc., has revealed low reliability because all optical elements of the optical system 290 joined by an adhesive were peeled off from each other at a thermal-cyclic reliability test.
The following is a possible reason for low reliability:
As already described, the color-separating and recombining optical system 290 has four PBSs 102, 103, 104 and 105 arranged such that their polarization-splitting planes 121, 131, 141 and 151 intersect each other almost like the character xe2x80x9cXxe2x80x9d.
In the reliability test, the optical elements were subjected to thermal expansion and contraction while the optical system 290 were being heated and cooled cyclically. Stress was then generated from the center of the intersection of the four PBSs in the direction of circumference due to thermal expansion and contraction. The circumferential stress could cause outward shear stress in heating whereas tensile stress in cooling at each joint section of the optical elements, thus resulting is peeling-off for the optical elements from each other.
The character-xe2x80x9cXxe2x80x9d-like arrangements of the PBSs 102, 103, 104 and 105 also poses the following problem:
As illustrated in FIG. 2, some components of light incident to the first PBS (light-incident-side PBS) 102 are further incident to the fourth PBS (light-emitting-side PBS) 105. The unnecessary light components L are projected onto a screen (not shown) via a projection lens 191, to generate bright portions on the screen, thus resulting in low quality for images displayed thereon. The image quality will be further lowered when the four PBSs 102, 103, 104 and 105 are bonded each other by a joint material 110 such as a transparent adhesive.
When the color-separating and -recombining-optical system 290 has integrators on reflective SLMs 161, 162 and 163 at the light-source side, an integrator-segment image is displayed on screen while light is illuminating these SLMs. However, light components spread over the periphery of each reflective SLM could also become the unnecessary light components L projected onto the screen.
In addition, light components reflected from the reflective SLMs 161, 162 and 163 could be reflected again at a first polarizing plate 181 and become the unnecessary light components L projected onto the screen.
Under consideration of the problems discussed above, a purpose of the present invention is to provide a low-cost but highly-reliable color-separating and -recombining optical system having polarization beam splitters unsusceptible to birefringence, suitable for use in reflective projection display.
Another purpose of the present invention is to provide a high-image-quality color-separating and -recombining optical system with a function of preventing unnecessary light components being displayed on a screen and a projection display using this optical system.
The present invention provides a color-separating and -recombining optical system comprising: cubic- or square column-like first to fourth polarization beam splitters having polarization-splitting planes intersecting each other like a character-xe2x80x9cXxe2x80x9d; and wavelength-selective polarizing converters each for rotating the plane of polarization of a specific-color light component by 90 degrees, one of the converters being placed at a light-incident side of the first splitter, another of the converters being placed at a light-emitting side of the fourth splitter, the first and the fourth splitters being provided at a light-incident side and a light-emitting side, respectively, of the optical system, the first and the fourth splitters being arranged as diagonally opposing each other, and the remaining converters being placed between at least two inner facing planes of the first to the fourth splitters, wherein at least the remaining converters and three of the first to the fourth splitters are joined each other to form an optical joint component with a gap between the remaining one splitter.
Moreover, the present invention provides a color-separating and -recombining optical system comprising: cubic- or square column-like first to fourth polarization beam splitters having polarization-splitting planes intersecting each other like a character-xe2x80x9cXxe2x80x9d; and wavelength-selective polarizing converters each for rotating the plane of polarization of a specific-color light component by 90 degrees, one of the converters being placed at a light-incident side of the first splitter, another of the converters being placed at a light-emitting side of the fourth splitter, the first and the fourth splitters being provided at a light-incident side and a light-emitting side, respectively, of the optical system, the first and the fourth splitters being arranged as diagonally opposing each other, and the remaining converters being placed between at least two inner facing planes of the first to the fourth splitters, wherein opto-elastic constants for the first to the fourth splitters have a relationship Ki less than Km and Ko, Ki and Km less than Ko or Ki less than Km less than Ko in which Ki, Km and Ko denote the opto-elastic constants for the first splitter, the second and the third splitters and the fourth splitter, respectively.
Moreover, the present invention provides a color-separating and -recombining optical system comprising: cubic- or square column-like first to fourth polarization beam splitters having polarization-splitting planes intersecting each other like a character-xe2x80x9cXxe2x80x9d; wavelength-selective polarizing converters each for rotating the plane of polarization of a specific-color light component by 90 degrees, one of the converters being placed at a light-incident side of the first splitter, another of the converters being placed at a light-emitting side of the fourth splitter, the first and the fourth splitters being provided at a light-incident side and a light-emitting side, respectively, of the optical system, the first and the fourth splitters being arranged as diagonally opposing each other, and the remaining converters being placed between at least two inner facing planes of the first to the fourth splitters; and a light blockage provided at an intersection of the polarization-splitting planes and surrounded by the first to the fourth splitters, the light blockage preventing light leakage from the first to the fourth splitters.
Furthermore, the present invention provides a color-separating and -recombining optical system comprising: cubic- or square column-like first to fourth polarization beam splitters having polarization-splitting planes intersecting each other like a character-xe2x80x9cXxe2x80x9d; wavelength-selective polarizing converters each for rotating the plane of polarization of a specific-color light component by 90 degrees, one of the converters being placed at a light-incident side of the first splitter, another of the converters being placed at a light-emitting side of the fourth splitter, the first and the fourth splitters being provided at a light-incident side and a light-emitting side, respectively, of the optical system, the first and the fourth splitters being arranged as diagonally opposing each other, and the remaining converters being placed between at least two inner facing planes of the first to the fourth splitters; and first light blockages provided at a first cornet of the cubic- or square column-like first splitter and a second corner of the cubic- or square column-like fourth splitter, edges of the first and the second corners diagonally opposing each other being cut off to be flat to face each other, the first light blockages preventing light leakage from the first to the fourth splitters.
Moreover, the present invention provides a projection display comprising: a light source for emitting unlinearly-polarized light; a first polarizer to allow only a first specific-linearly-polarized light component of the unlinearly-polarized light to pass therethrough; a color-separating and -recombining optical system including cubic- or square column-like first to fourth polarization beam splitters having polarization-splitting planes intersecting each other like a character-xe2x80x9cXxe2x80x9d, the first splitter being provided as facing the first polarizer, and wavelength-selective polarizing converters each for rotating the plane of polarization of a specific-color light component by 90 degrees, one of the converters being placed at a light-incident side of the first splitter, another of the converters being placed at a light-emitting side of the fourth splitter, the first and the fourth splitters being provided at a light-incident side and a light-emitting side, respectively, of the optical system, the first and the fourth splitters being arranged as diagonally opposing each other, and the remaining converters being placed between at least two inner facing planes of the first to the fourth splitters, wherein at least the remaining converters and three of the first to the fourth splitters are joined each other to form an optical joint component with a gap between the remaining one splitter; reflective spatial light modulators for light modulation in accordance with a video signal, provided outside the optical system, as facing each light-passing plane of the second and the third splitters, a second polarizer provided as facing a light-emitting side plane of the fourth splitter, to allow only a second specific-linearly-polarized light component emitted from the light-emitting side plane of the fourth splitter to pass therethrough; and a projection lens provided as facing the second polarizer, to receive the second specific-linearly-polarized light component for image projection.
Furthermore, the present invention provides a projection display comprising: a light source for emitting unlinearly-polarized light; a first polarizer to allow only a first specific-linearly-polarized light component of the unlinearly-polarized light to pass therethrough; a color-separating and -recombining optical system including cubic or square column-like first to fourth polarization beam splitters having polarization-splitting planes intersecting each other like a character-xe2x80x9cXxe2x80x9d, the first splitter being provided as facing the first polarizer, and wavelength-selective polarizing converters each for rotating the plane of polarization of a specific-color light component by 90 degrees, one of the converters being placed at a light-incident side of the first splitter, another of the converters being placed at a light-emitting side of the fourth splitter, the first and the fourth splitters being provided at a light-incident side and a light-emitting side, respectively, of the optical system, the first and the fourth splitters being arranged as diagonally opposing each other, and the remaining converters being placed between at least two inner facing planes of the first to the fourth splitters, wherein opto-elastic constants for the first to the fourth splitters have a relationship Ki less than Km and Ko, Ki and Km less than Ko or Ki less than Km less than Ko in which Ki, Km and Ko denote the opto-elastic constants for the first splitter, the second and the third splitters and the fourth splitter, respectively; reflective spatial light modulators for light modulation in accordance with a video signal, provided outside the optical system, as facing each light-passing plane of the second and the third splitters, a second polarizer provided as facing a light-emitting side plane of the fourth splitter, to allow only a second specific-linearly-polarized light component emitted from the light-emitting side plane of the fourth splitter to pass therethrough; and a projection lens provided as facing the second polarizer, to receive the second specific-linearly-polarized light component for image projection.
Furthermore, the present invention provides a projection display comprising: a light source for emitting unlinearly-polarized light; a first polarizer to allow only a first specific-linearly-polarized light component of the unlinearly-polarized light to pass therethrough; a color-separating and -recombining optical system including cubic- or square column-like first to fourth polarization beam splitters having polarization-splitting planes intersecting each other like a character-xe2x80x9cXxe2x80x9d, the first splitter being provided as facing the first polarizer, wavelength-selective polarizing converters each for rotating the plane of polarization of a specific-color light component by 90 degrees, one of the converters being placed at a light-incident side of the first splitter, another of the converters being placed at a light-emitting side of the fourth splitter, the first and the fourth splitters being provided at a light-incident side and a light-emitting side, respectively, of the optical system, the first and the splitters being arranged as diagonally opposing each other, and the remaining converters being placed between at least two inner facing planes of the first to the fourth splitters, and a light blockage provided at an intersection of the polarization-splitting planes and surrounded by the first to the fourth splitters, the light blockage preventing light leakage from the first to the fourth splitters; reflective spatial light modulators for light modulation in accordance with a video signal, provided outside the optical system, as facing each light-passing plane of the second and the third splitters, a second polarizer provided as facing a light-emitting side plane of the fourth splitter, to allow only a second specific-linearly-polarized light component emitted from the light-emitting side plane of the fourth splitter to pass therethrough; and a projection lens provided as facing the second polarizer, to receive the second specific-linearly-polarized light component for image projection.
Furthermore, the present invention provides a projection display comprising: a light source for emitting unlinearly-polarized light; a first polarizer to allow only a first specific-linearly-polarized light component of the unlinearly-polarized light to pass therethrough; a color-separating and -recombining optical system including cubic- or square column-like first to fourth polarization beam splitters having polarization-splitting planes intersecting each other like a character-xe2x80x9cXxe2x80x9d, the first splitter being provided as facing the first polarizer, wavelength-selective polarizing converters each for rotating the plane of polarization of a specific-color light component by 90 degrees, one of the converters being placed at a light-incident side of the first splitter, another of the converters being placed at a light-emitting side of the fourth splitter, the first and the fourth splitters being provided at a light-incident side and a light-emitting side, respectively, of the optical system, the first and the splitters being arranged as diagonally opposing each other, and the remaining converters being placed between at least two inner facing planes of the first to the fourth splitters, and light blockages provided at a first corner of the cubic- or square column-like first splitter and a second corner of the cubic- or square column-like fourth splitter, edges of the first and the second corners diagonally opposing each other being cut off to be flat to face each other, the light blockages preventing light leakage from the first to the fourth splitters; reflective spatial light modulators for light modulation in accordance with a video signal, provided outside the optical system, as facing each light-passing plane of the second and the third splitters, a second polarizer provided as facing a light-emitting side plane of the fourth splitter, to allow only a second specific-linearly-polarized light component emitted from the light-emitting side plane of the fourth splitter to pass therethrough; and a projection lens provided as facing the second polarizer, to receive the second specific-linearly-polarized light component for image projection.